Properties of Solutions – General Chemistry Study Notes (Chapter 13)

Properties of Solutions

The Solution Process

Solutions are homogeneous mixtures of two or more substances. In a solution, the solute is dispersed uniformly throughout the solvent. The ability of substances to form solutions depends on two main factors: the natural tendency toward mixing and the nature of intermolecular forces.

• Solute: The substance present in a lesser amount, which is dissolved.

• Solvent: The substance present in a greater amount, which does the dissolving.

• Homogeneous mixture: A mixture with uniform composition throughout.

Example: Dissolving copper(II) sulfate (CuSO4) in water forms a blue solution, with CuSO4 as the solute and water as the solvent.

Natural Tendency Toward Mixing

The mixing of gases and liquids is often a spontaneous process, driven by the tendency to increase randomness (disorder) in the system. This increase in randomness is measured by a thermodynamic quantity called entropy (S).

• Each gas in a mixture acts independently to fill the container.

• Mixing increases entropy, favoring the formation of solutions.

Example: When O2 and Ar gases are mixed, the molecules become more randomly distributed, increasing entropy.

Intermolecular Forces of Attraction

The formation of solutions depends on the types and strengths of intermolecular forces between solute and solvent molecules. Several types of intermolecular forces can operate:

• Dispersion forces: Present in all molecules, especially significant in nonpolar molecules (e.g., pentane).

• Dipole-dipole interactions: Occur between polar molecules (e.g., acetone and chloroform).

• Hydrogen bonding: A strong type of dipole-dipole interaction, present when H is bonded to N, O, or F (e.g., ethanol and water).

• Ion-dipole interactions: Occur between ions and polar molecules (e.g., Na+ and H2O).

Additional info: The strength and type of intermolecular forces determine the solubility of substances in various solvents.

The Effect of Intermolecular Forces on Solution Formation

Three types of intermolecular attractions are involved in the formation of a solution:

• Solute–solute interactions: Must be overcome to disperse solute particles through the solvent.

• Solvent–solvent interactions: Must be overcome to make room for solute particles in the solvent.

• Solvent–solute interactions: Occur as solute and solvent particles mix, stabilizing the solution.

Example: When NaCl dissolves in water, the ionic bonds between Na+ and Cl− are overcome, and new ion-dipole interactions form between the ions and water molecules.